Transition Metal Hexacyanoferrate Battery with Carbonaceous Anode

ABSTRACT

A transition metal hexacyanoferrate (TMH) cathode battery is provided. The battery has a A x Mn[Fe(CN) 6 ] y .zH 2 O cathode, where the A cations are either alkali or alkaline-earth cations, such as sodium or potassium, where x is in the range of 1 to 2, where y is in the range of 0.5 to 1, and where z is in the range of 0 to 3.5. The A x Mn[Fe(CN) 6 ] y .zH 2 O has a rhombohedral crystal structure with Mn 2+/3+  and Fe 2+/3+  having the same reduction/oxidation potential. The battery also has an electrolyte, and anode made of an A metal, an A composite, or a material that can host A atoms. The battery has a single plateau charging curve, where a single plateau charging curve is defined as a constant charging voltage slope between 15% and 85% battery charge capacity. Fabrication methods are also provided.

RELATED APPLICATIONS

This application is a Continuation of an application entitled,TRANSITION METAL HEXACYANOFERRATE BATTERY CATHODE WITH SINGLE PLATEAUCHARGE/DISCHARGE CURVE, invented by Yuhao Lu et al., Ser. No.14/744,476, filed Jun. 19, 2015, attorney docket No. SLA3265.1;

which is a Divisional of an application entitled, TRANSITION METALHEXACYANOFERRATE BATTERY CATHODE WITH SINGLE PLATEAU CHARGE/DISCHARGECURVE, invented by Yuhao Lu et al., Ser. No. 13/752,930, filed Jan. 29,2013, attorney docket No. SLA3265;

which is a Continuation-in-Part of an application entitled,SUPERCAPACITOR WITH HEXACYANOMETALLATE CATHODE, ACTIVATED CARBON ANODE,AND AQUEOUS ELECTROLYTE, invented by Yuhao Lu et al., Ser. No.13/603,322, filed Sep. 4, 2012, attorney docket No. SLA3212;

Ser. No. 13/752,930 is a Continuation-in-Part of an applicationentitled, IMPROVEMENT OF ELECTRON TRANSPORT IN HEXACYANOMETALLATEELECTRODE FOR ELECTROCHEMICAL APPLICATIONS, invented by Yuhao Lu et al.,Ser. No. 13/523,694, filed Jun. 14, 2012, now U.S. Pat. No. 8,957,796,with an issue date of Feb. 17, 2015;

which is a Continuation-in-Part of a pending application entitled,ALKALI AND ALKALINE-EARTH ION BATTERIES WITH HEXACYANOMETALLATE CATHODEAND NON-METAL ANODE, invented by Yuhao Lu et al., Ser. No. 13/449,195,filed Apr. 17, 2012, attorney docket no. SLA3151;

which is a Continuation-in-Part of a pending application entitled,ELECTRODE FORMING PROCESS FOR METAL-ION BATTERY WITH HEXACYANOMETALLATEELECTRODE, invented by Yuhao Lu et al., Ser. No. 13/432,993, filed Mar.28, 2012, attorney docket no. SLA3146. All these applications areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to electrochemical cells and, moreparticularly, to a transition-metal hexacyanoferrate (TMH) cathodebattery and associated fabrication processes.

2. Description of the Related Art

A battery is an electrochemical cell through which chemical energy andelectric energy can be converted back and forth. The energy density of abattery is determined by its voltage and charge capacity. Lithium hasthe most negative potential of −3.04 V vs. H₂/H⁺, and has the highestgravimetric capacity of 3860 milliamp-hours per gram (mAh/g). Due totheir high energy densities, lithium-ion batteries have led the portableelectronics revolution. However, the high cost of lithium metal rendersdoubtful the commercialization of lithium batteries as large scaleenergy storage devices. Further, the demand for lithium and its reserveas a mineral have raised the need to build other types metal-ionbatteries as an alternative.

Lithium-ion (Li-ion) batteries employ lithium storage compounds as thepositive (cathode) and negative (anode) electrode materials. As abattery is cycled, lithium ions (Li⁺) are exchanged between the positiveand negative electrodes. Li-ion batteries have been referred to asrocking chair batteries because the lithium ions “rock” back and forthbetween the positive and negative electrodes as the cells are chargedand discharged. The positive electrode (cathode) material is typically ametal oxide with a layered structure, such as lithium cobalt oxide(LiCoO₂), or a material having a tunneled structure, such as lithiummanganese oxide (LiMn₂O₄), on an aluminum current collector. Thenegative electrode (anode) material is typically a graphitic carbon,also a layered material, on a copper current collector, in thecharge-discharge process, lithium ions are inserted into, or extractedfrom interstitial spaces of the active materials.

Similar to the lithium-ion batteries, metal-ion batteries use themetal-ion host compounds as their electrode materials in whichmetal-ions can move easily and reversibly. As for a Li⁺-ion, it has oneof the smallest radii of all metal ions and is compatible with theinterstitial spaces of many materials, such as the layered LiCoO₂,olivine-structured LiFePO₄, spinel-structured LiMn₂O₄, and so on. Othermetal ions, such as Na⁺, K⁺, Mg²⁺, Al³⁺, Zn²⁺, etc., with large sizes,severely distort Li-based intercalation compounds and ruin theirstructures in several charge/discharge cycles. Therefore, new materialswith large interstitial spaces would have to be used to host suchmetal-ions in a metal-ion battery.

Transition-metal hexacyanoferrates (TMHs) have been investigated as thecathode materials in lithium-ion batteries (LIBs) [1, 2] because theyaccommodate lithium-ion intercalation in their interstitial spaces.However, the lithium-ion size is too small to match the spaces, whichdegrades the TMH capacities rapidly during lithium-ion intercalation. In2004, Eftekhari [3] used iron hexacyanoferrate (Prussian blue) as thecathode material in potassium-ion batteries (KIBs) with a counterelectrode of potassium metal. The organic electrolyte was 1M KBF₄ in 3:7ethylene carbonate/ethylmethyl carbonate (wt.). The size ofpotassium-ion is almost two times that of the lithium-ions, and matchesthe interstitial spaces of Prussian blue very well. The results showedthat Prussian blue was a good electrode material for KIBs, demonstratinga reversible capacity of ca. 75 mAh/g and a good capacity retention.

Similarly, Cui's group studied the intercalation behavior of large ions,for example, sodium, potassium and ammonium ions, in copper (CuHCF) andnickel hexacyanoferrates (NiHCF) with an aqueous electrolyte [4-6].These large size ions were compatible with the interstitial spaces ofthe hexacyanoferrates, so that CuHCF and NiHCF demonstrated goodcapacity retention. Due to the narrow electrochemical window of water,these materials were evaluated under low voltages and demonstrated lowenergy density. In order to improve the performance, organicelectrolytes with a wide electrochemical window would have to be used toincrease the operation voltages of the TMH electrodes.

Goodenough's group [7] investigated a series of Prussian blue analoguesin sodium-ion batteries (SIBs) with organic electrolytes. They foundthat KFe(II)Fe(III)(CN)₆ demonstrated the highest capacity of 95 mAh/g,and KMnFe(CN)₆, KNiFe(CN)₆, KCuFe(CN)₆, and KCoFe(CN)₆ had a capacity of50˜70 mAh/g. In the first 20 cycles, the capacity retention ofKFeFe(CN)₆ was higher than 97%.

FIG. 1 is a diagram depicting the crystal structure of atransition-metal hexacyanoferrate (TMH) in the form of A_(x)M1M2(CN)₆(prior art). TMHs have an open framework. The large tetrahedrallycoordinated A sites can host alkali, alkaline ions (A_(x)), and H₂Omolecules. The number of alkali or alkaline ions in the large cages ofthis crystallographically porous framework may vary from x=0 to x=2,depending on the valence of M1 and M2 that are metal ions. Of course, asthe electrode materials in SIBs or KIBs, TMHs are expected to have twoof Na⁺- or K⁺-ions in their interstitial spaces. Therefore, M1 and M2with valences of +2 are selected in the synthesis process to produce(Na,K)₂M1M2(CN)₆. Moreover, M1 and M2 can be reversibly oxidized andreduced between the valences of +2 and +3 when Na⁺- or K⁺-ions areextracted/inserted from/into TMHs. When these TMHs are used as theelectrode materials in SIBs or KIBs, it is hard to obtain very smoothand flat charge/discharge curves due to the fact that M1 and M2 havedifferent chemical potentials, or occupy different spin states. Forexample, Na₂CoFe(CN)₆ exhibited two plateaus in its discharge curves at3.78 V and 3.28 V vs. Na/Na⁺, that correspond to the reduction of Co³⁺and Fe³⁺, respectively [8].

In order to obtain cheap electrode materials for batteries, manganese isa good choice for TMHs, e.g., (Na,K)_(x),Mn[Fe(CN)₆]_(y).zH₂O. Matsudaand Moritomo [9] synthesized a Na_(1.32)Mn[Fe(CN)₆].3.5H₂O film for alithium-ion battery that showed three plateaus in its discharge curve,and two plateaus in its charge curve that could be explained by thereduction and oxidation of Mn and Fe in the material. In the battery,the materials showed a capacity of 128 mAh/g. In a sodium-ion battery,Goodenough's group [7] also reported multiple plateaus in itscharge/discharge curves. The Mn-based TMHs demonstrated a capacity of 70mAh/g. However, it would be better to have a single, rather than twovoltage plateaus in the charge/discharge curves. A battery with a singleplateau charge/discharge curve has a tighter (more uniform)charge/discharge voltage than a battery with multiple plateaus. Atighter charge/discharge voltage renders a simpler battery control.

FIGS. 11A and 11B are graphs depicting the electrochemical behavior of asynthesized Na_(x)Mn[Fe(CN)₆]_(y).zH₂O cathode in sodium-ion batteries(prior art). When sodium ions electrochemically move in and out of theinterstitial space of magnesium hexacyanoferrate (MnHCF), two mainpotentials appear in the charge or discharge process, due to the redoxreaction of Mn and Fe [7, 9] in MnHCF. In battery applications, theredox reaction of MnHCF causes two plateaus during charge/discharge. Twoplateaus are observed during charge/discharge that correspond to theredox reaction of Mn at higher voltages and the redox reaction of Fe atlow voltages.

-   -   [1] V. D. Neff, Some performance characteristics of a Prussian        Blue battery, Journal of Electrochemical Society, 132 (1985)        1382-1384.    -   [2] N. Imanishi, T. Morikawa, J. Kondo, Y. Takeda, O.        Yamamoto, N. Kinugasa, T. Yamagishi, Lithium intercalation        behavior into iron cyanide complex as positive electrode of        lithium secondary battery, Journal of Power Sources, 79 (1999)        215-219.    -   [3] Ali Eftekhari., Potassium secondary cell based on Prussian        blue cathode, Journal of Power Sources, 126 (2004) 221-228.    -   [4] Colin D. Wessells, Rober A. Huggins, Yi Cui, Copper        hexacyanoferrate battery electrodes with long cycle life and        high power, Nature Communication, 2(2011) 550.    -   [5] Colin D. Wessells, Sandeep V. Peddada, Robert A. Huggins, Yi        Cui, Nickel hexacyanoferrate nanoparticie electrodes for aqueous        sodium and potassium ion batteries. Nano Letters, 11(2011)        5421-5425.    -   [6] Colin D. Wessells, Sandeep V. Peddada, M. T. McDowell,        Robert A. Huggins, Yi Cui, The effect of insertion species on        nanostructured open framework hexacyanoferrate battery        electrodes, Journal of Electrochemical Society, 159 (2012)        A98-A103.    -   [7] Yuhao Lu, Long Wang, John B. Goodenough, Prussian blue: a        new framework for sodium batteries, Chemistry Communication,        48(2012)6544-6546.    -   [8] J. F. Qian, M. Zou, Y. L. Cao, H. X. Yang,        Na_(x)M_(y)Fe(CN)₆ (M=Fe, Co, Ni): A New class of cathode        Materials for sodium Ion batteries, Journal of Electrochemistry        (Chinese), 18(2012)108-112.    -   [9] T. Matsuda, Y. Moritomo, Thin film electrode of Prussian        blue analogue for Li-ion battery, Applied Physics Express,        4(2011)047101.

It would be advantageous if a TMH cathode battery could be made tooperate with a single plateau charge and discharge curve.

SUMMARY OF THE INVENTION

Disclosed herein are processes to prepare transition-metalhexacyanoferrate (TMH), such as (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O forsodium-ion batteries (SIBs) or potassium-ion batteries (KIBs). Threefactors are associated with the improved processes. Firstly, reductiveagents may be added into the synthesis solution to protect Mn²⁺ and Fe²⁺from oxidation, so that more Na⁺- or K⁺-ions can be kept in theinterstitial spaces of (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O. Secondly, theproduct may be vacuum-dried at a temperature range from 200° C. to 2000°C., regardless of whether it has been previously dried in air. Lastly,electronic conductors, for example, carbonaceous materials, can bedispersed into the reaction solution to improve performance of(Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O as electrode materials in SIBS or KIBs.The process can obtain (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O with the followingperformance:

(1) Just single plateau appears the charge/discharge curves.

(2) The charge/discharge curves are smooth and flat.

(3) The material exhibits a high capacity, energy efficiency, andcoulombic efficiency.

Accordingly, a TMH cathode battery is provided. The battery has aA_(x)Mn[Fe(CN)₆]_(y).zH₂O cathode, where the A cations are either alkalior alkaline-earth cations, where x is in the range of 1 to 2, where y isin the range of 0.5 to 1, and where z is in the range of 0 to 3.5. TheA_(x)Mn[Fe(CN)₆].zH₂O has a rhombohedral crystal structure withMn^(2+/3+) and Fe^(2+/3+) having the same reduction/oxidation potential.The battery also has an electrolyte, and an anode made of an A metal, anA composite, or a material that can host A atoms. The electrolyte may bean organic solvent containing an A-atom salt. In one aspect, A is eithersodium (Na) or potassium (K).

The battery has a single plateau charging curve, where a single plateaucharging curve is defined as a constant charging voltage slope between15% and 85% battery charge capacity. Likewise, the battery has a singleplateau discharge curve, where a single plateau discharge curve isdefined as a constant discharge voltage slope between 85% and 15%battery charge capacity.

Additional details of the above-described battery, battery cathode,battery fabrication, and battery usage are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram depicting the crystal structure of atransition-metal hexacyanoferrate (TMH) in the form of A_(x)M1M2(CN)₆(prior art).

FIG. 2A is a diagram depicting an A_(x)MnFe(CN)₆ crystal structure, asan example of a TMH battery cathode material.

FIG. 2B is a diagram depicting a rhombohedral crystal structure.

FIG. 3 is a partial cross-sectional view of a TMH cathode battery.

FIG. 4 is a diagram depicting charging and discharging curves associatedwith the battery of FIG. 3.

FIGS. 5A and 5B are X-ray diffraction (XRD) patterns contrasting,respectively, (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O dried in air, with vacuumdrying.

FIGS. 6A and 6B are graphs contrasting the electrochemical behavior ofNa_(x)Mn[Fe(CN)₆]_(y).zH₂O dried in different conditions.

FIGS. 7A and 7B respectively depict the charge/discharge profile of thecomposite of graphene oxide graphene) and Na_(x)Mn[Fe(CN)₆]_(y).zH₂O,and a chronoamperogram of the composite of graphene oxide (or graphene)and Na_(x)Mn[Fe(CN)₆]_(y).zH₂O at the second cycle.

FIG. 8 is a flowchart illustrating a method for synthesizing a TMHbattery material.

FIG. 9 is a flowchart illustrating a method for fabricating a TMHbattery cathode electrode.

FIG. 10 is a flowchart illustrating a method for using a TMH battery.

FIGS. 11A and 11B are graphs depicting the electrochemical behavior of asynthesized Na_(x)Mn[Fe(CN)₆]_(y).zH₂O cathode in sodium-ion batteries(prior art).

DETAILED DESCRIPTION

FIG. 2A is a diagram depicting an A_(x)MnFe(CN)₆ crystal structure, asan example of a TMH battery cathode material. The cathode comprisesA_(x)Mn[Fe(CN)₆]_(y).zH₂O;

where A cations are alkali or alkaline-earth cations;

where x is in the range of 1 to 2;

where y is in the range of 0.5 to 1; and,

where z is in the range of 0 to 3.5.

The A_(x)MnFe[(CN)₆]_(y).zH₂O has a rhombohedral crystal structure withMn^(2+/3+) and Fe^(2+/3+) having the same reduction/oxidation potential.In one aspect, the A cations nay be either sodium (Na) or potassium (K).

FIG. 2B is a diagram depicting a rhombohedral crystal structure. Thisstructure is just a little bit “twisted” from a cubic crystal structure(α=β=γ=90°). Although the rhombohedral crystal structure cannotphysically be seen under a microscope, it can be detected using X-raydiffraction, see FIGS. 5A and 5B.

FIG. 3 is a partial cross-sectional view of a TMH cathode battery. Thebattery 300 comprises a A_(x)Mn[Fe(CN)₆]_(y).zH₂O cathode 302, where Acations are either alkali or alkaline-earth cations, where x is in therange of 1 to 2, where y is in the range of 0.5 to 1 and where z is therange of 0 to 3.5, see FIG. 2A. The A_(x)Mn[Fe(CN)₆]_(y).zH₂O has arhombohedral crystal structure, with Mn^(2+/3+) and Fe^(2+/3+) havingthe same reduction/oxidation potential. The battery 300 has an anode 304made from an A metal, an A composite, or a material that can host Aatoms. The battery 300 further comprises an electrolyte 306. Theelectrolyte may, for example, be an organic solvent containing A-atomsalts. In one aspect, A is either Na or K.

FIG. 4 is a diagram depicting charging and discharging curves associatedwith the battery of FIG. 3. As shown, the battery has a single plateaucharging curve, where a single plateau charging curve is defined as aconstant charging voltage slope between 15% and 85% battery chargecapacity. Likewise, the battery has a single plateau discharge curve,where a single plateau discharge curve is defined as a constantdischarge voltage slope between 85% and 15% battery charge capacity. Itshould be understood that the 15% and 85% battery charge capacity valuesmay vary depending upon a particular design, and that in some instances,the range may extend to less than 15% battery charge capacity, and/orgreater than 85%. Alternatively stated, the battery has a single plateaucharging/discharging curve, where a single plateau charging/dischargingcurve is defined by the derivative of charge capacity to voltage (dQ/dV)of a completed charging/discharging curve having only one peak. In oneaspect, the above-mentioned single plateau curves are associated with abattery having a discharge capacity of greater than 90 milliamp hoursper gram (mAh/g). Alternatively stated, when there are two A atoms in alattice (x=2), and all TMH materials are active (meaning they can chargeand discharge), the discharge capacity associated with the first A atomis 90 mAh/g capacity. The discharge of the second A atom in the latticeincreases the capacity to a value greater than 90 mAh/g.

As explained above, sodium or potassium (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂Ocan be fabricated for sodium-ion batteries (SIBs) or potassium-ionbatteries (KIBs) that demonstrate a single plateau duringcharge/discharge. In addition, these batteries have a high capacity. Thepreparation described herein (1) increases the value of x in(Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O to obtain high capacity; (2) reduces thevalue of z in (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O; and, (3) enhances theelectronic conductivity of (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O.

Precipitation is a simple method to synthesize(Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O. A Mn²⁺ solution and a Fe(CN)₆ ⁴⁻solution are mixed, and (Na,K)1 _(x)Mn[Fe(CN)₆]_(y).zH₂O precipitatesimmediately. When the process is exposed to air, it is inevitable thatMn²⁺- or Fe²⁺-ions are oxidized to Mn³⁺ or Fe³⁺. The oxidation reducesthe content of Na⁺ or K⁺ in the final product of(Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O because all charges should be neutralizedin the material. As electrode materials, the small content of Na⁺ or K⁺means a small capacity of (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O for SIBs orKIBs. In order to prevent the oxidation in the synthesis, reductiveagents can be added into both the Mn²⁺ solution, the Fe(CN)₆ ⁴⁻solution, or both, at the beginning of the process. The reductive agentsmay, for example, be sodium borohydride (NaBH₄) and hydrazine (N₂ ₄),but other agents would be apparent to those with skill in the art. Inthe solution of Mn²⁺, the concentration of reductive agents is from 0 to100 moles/liter. In the solution of Fe(CN)₆ ⁴⁻, the concentration ofreductive agents is from 0 to 100 in moles/liter. After the reactionfinishes, the precipitate is collected and washed to obtain(Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O.

FIGS. 5A and 5B are X-ray diffraction (XRD) patterns contrasting,respectively, (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O dried in air, with vacuumdrying. The XRD pattern of FIG. 5A is associated with a cubic crystalstructure and the XRD pattern of FIG. 5B is associated with arhombohedral crystal pattern. (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O can bedried in either air or in a vacuum. The crystal structure andperformance of the (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O is highly dependentupon drying condition. Na_(x)Mn[Fe(CN)₆]_(y).zH₂O dried in air (FIG. 5A)has a cubic structure, but it shows a structure change to a rhombohedralphase after being dried in a vacuum with a pressure of less than 0.1torr (FIG. 5B).

FIGS. 6A and 6B are graphs contrasting the electrochemical behavior ofNa_(x)Mn[Fe(CN)₆]_(y).zH₂O dried in different conditions. ANa_(x)Mn[Fe(CN)₆]_(y).zH₂O cathode was evaluated in standard CR2032 coincells that consisted of a sodium anode or hard carbon anode, separatedby a Celgard polypropylene separator. The Na_(x)Mn[Fe(CN)₆]_(y).zH₂Oelectrode was made of Na_(x)Mn[Fe(CN)₆]_(y).zH₂O, carbon black (SuperP), and polytetrafluoroethylene (PTFE) binder. The electrolyte wassaturated NaClO₄ in 1:1 ethylene carbonate/diethyl carbonate (EC/DEC)(vol.:vol.). The charge/discharge currents were 0.1 C 170 milliamp-hoursper gram (mAh/g). All cells were cycled between 2 V and 4.2 V.

After being dried in air, the Na_(x)Mn[Fe(CN)₆]_(y).zH₂O shows twoplateaus during charge and discharge (FIG. 6A). However, only oneplateau is observed in the charge/discharge curves after vacuum dryingwith a pressure of less than 0.1 torr (FIG. 6B). Moreover, the vacuumdrying process increases the energy density and efficiency ofNa_(x)Mn[Fe(CN)₆]_(y).zH₂O. The fact that a material with the samechemical formula of Na_(x)Mn[Fe(CN)₆]_(y).zH₂O may demonstrate differentcrystal structures and different charge/discharge profiles is a resultof different drying conditions.

In order to improve the performance of (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O inSIBs or KIBs further, manganese TMHs can be composited with electronicconductors. In the composite structure, small sized(Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O particles can be uniformly coated on thelarge surface area of the conductors, so that the electronicconductivity of the (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O electrode isimproved, and the utilization rate of the active(Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O material is improved. The highconductivity, uniform distribution of (Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O,and high utilization rate improves battery performance in terms ofcapacity and power. The electronic conductor may be metal powders,carbonaceous materials, and polymers, but is not limited to just theseexamples. With the vacuum drying process, a(Na,K)_(x)Mn[Fe(CN)₆]_(y).zH₂O conductor composite structured electrodeshows a single plateau during charge and discharge. Moreover, itscapacity is improved significantly due to the higher utilization rate.

A composite of graphene and Na_(x)Mn[Fe(CN)₆]_(y).zH₂O, as an example,is also useful in processing. Graphene oxide is ultrasonically dispersedinto distilled water. The graphene oxide solution can be put into theMn²⁺ solution, the Fe(CN)₆ ⁴⁻ solution, or both. Using the precipitationmethod with reductive agent, the graphene oxide is graphene and acomposite of graphene and Na_(x)Mn[Fe(CN)₆]_(y).zH₂O forms. Afterseparation and washing, the composite is dried in an oven at atemperature between 200° C. and 2000° C. The final product is acomposite Na_(x)Mn[Fe(CN)₆]_(y).zH₂O/graphene powder. If the drying isin an vacuum oven with pressure less than 0.1 torr, theNa_(x)Mn[Fe(CN)₆]_(y).zH₂O has a rhombohedral crystal structure.

FIGS. 7A and 7B respectively depict the charge/discharge profile of thecomposite of graphene oxide graphene) and Na_(x)Mn[Fe(CN)₆]_(y).zH₂O,and a chronoamperogram of the composite of graphene oxide (or graphene)and Na_(x)Mn[Fe(CN)₆]_(y).zH₂O at the second cycle. A vacuum dryingprocess was applied to the composite Na_(x)Mn[Fe(CN)₆]_(y).zH₂O/graphenepowder. The counter electrode is sodium metal and the organicelectrolyte is saturated NaClO₄ in EC/DEC. Graphene oxide in thesynthesis solution increases the capacity of Na_(x)Mn[Fe(CN)₆]_(y).zH₂Osignificantly. The vacuum drying process results in theNa_(x)Mn[Fe(CN)₆]_(y).zH₂O having single and very flat charge/dischargecurves (FIG. 7A), indicating that sodium ion intercalation in theelectrode is mostly likely a two-phase process, rather than the solidsolution behavior shown in FIG. 6A. The electrode's chronoamperogram(FIG. 7B) also demonstrates the two-phase process with a single sharppeak during charge/discharge.

FIG. 8 is a flowchart illustrating a method for synthesizing a TMHbattery material. Although the method is depicted as a sequence ofnumbered steps for clarity, the numbering does not necessarily dictatethe order of the steps. It should be understood that some of these stepsmay be skipped, performed in parallel, or performed without therequirement of maintaining a strict order of sequence. Generallyhowever, the method follows the numeric order of the depicted steps. Themethod starts at Step 800.

Step 802 prepares a first solution of Mn²⁺. Step 804 prepares a secondsolution of an A salt of Fe(CN)₆ ⁴⁻. Step 806 mixes the first and secondsolutions. In response to mixing the first and second solutions,A_(x)Mn[Fe(CN)₆]_(y).zH₂O is precipitated in Step 808. The A cations arealkali or alkaline-earth cations, such as Na or K. The variable x is inthe range of 1 to 2, y is in the range of 0.5 to 1, and z is in therange of 0 to 3.5. Step 810 dries the precipitatedA_(x)Mn[Fe(CN)₆]_(y).zH₂O in a vacuum oven with a pressure of less than0.1 torr. In some aspect, the pressure is less than 0.01 torr. Forexample, the drying temperature may be in the range of 20° C. to 200° C.In one aspect, Step 808 forms the A_(x)Mn[Fe(CN)₆]_(y).zH₂O in a cubicstructure and Step 812 forms the A_(x)Mn[Fe(CN)₆]_(y).zH₂O with arhombohedral crystal structure, with Mn^(2+/3+) and Fe^(2+/3+) havingthe same reduction/oxidation potential.

During the synthesis process, it is inevitable that water moleculesoccupy the large interstitial spaces of TMHs. Water molecules in TMHscause the A ions to move from the center of the elementary cubes, whichmakes the A ions interact with the two transition-metal redox couplesdifferently. That is, the A-ions are located in two chemicalenvironments. The charge/discharge, or extraction/insertion of A ions intwo chemical environments correspond to the two charge/dischargeplateaus seen in FIG. 11A. Under a vacuum with less than 0.1 torrpressure, water molecules are removed from the TMH interstitial spaces,permitting A-ions go to the center of the cubes. All A-ions then have asingle, identical chemical environment, which causes just one plateauduring charge/discharge. Further, the cubic structure of TMHs is changedto rhombohedral due to the water removal. It is still possible forrhombohedral TMHs to exhibit two plateaus during charge/discharge if theA-ions do not occupy the center positions due to the water molecules inTMHs. However, the water removal from the crystal results in a phasetransformation from cubic to rhombohedral, and also is the root cause ofthe one-plateau behavior of TMHs described herein. In other words, theuse of higher vacuum pressure results in the more effective removal ofwater from the TMH crystal structure, and the single plateaucharge/discharge curves.

In one aspect, prior to mixing the first and second solutions in Step806, Step 805 b adds reductive agents to the first solution, the secondsolution, or both the first and second solutions. Some exemplaryreductive agents include sodium borohydride (NaBH₄) hydrazine (N₂H₄),and a combination of NaBH₄ and N₂H₄. For example, Step 805 b may add thereductive agents to the first solution in a concentration in a range of0 to 100 moles/liter. Likewise, the reductive agents may be added to thesecond solution in a concentration of 0 to 100 moles/liter.

In a different aspect, Step 805 a ultrasonically disperses carbonaceousmaterials in distilled water, creating a third solution. Then, Step 806additionally mixes the third solution with the first solution, secondsolution, or both the first and second solutions, and precipitating theA_(x)Mn[Fe(CN)₆]_(y).zH₂O in Step 808 includes precipitating a compositeof carbonaceous materials and A_(x)Mn[Fe(CN)₆]_(y).zH₂O. Some examplesof carbonaceous materials include graphene oxide, partially reducedgraphene oxide, graphene, carbon black, and graphite. If thecarbonaceous material used in Step 805 a is graphene oxide, then Step805 b may be performed by adding a reducing agent such as NaBH₄, N₂H₄,or both NaBH₄ and N₂H₄ to the mixture of the first, second, and thirdsolutions.

FIG. 9 is a flowchart illustrating a method for fabricating a TMHbattery cathode electrode. The method begins at Step 900. Step 902 mixesTMH material with a conducting carbon and an organic binder in anorganic solution, creating a A_(x)Mn[Fe(CN)₆]_(y).zH₂O paste. The Acations are alkali or alkaline-earth cations, such as Na and K. Thevariable x is in the range of 1 to 2, y is in the range of 0.5 to 1, andz is in the range of 0 to 3.5. Step 904 forms A_(x)Mn[Fe(CN)₆]_(y).zH₂Omaterial on a current collector to create an electrode. The currentcollector may, for example, be aluminum (Al), titanium (Ti), orstainless steel. Step 906 dries the electrode. Step 908 supplies theA_(x)Mn[Fe(CN)₆]_(y).zH₂O electrode with rhombohedral crystal structure,with Mn^(2+/3+) and Fe^(2+/3+) having the same reduction/oxidationpotential.

In one aspect, the TMH material used in Step 902 is the end product(Step 812) of the method described in FIG. 8, in which case TMH materialhas already been vacuum dried at a pressure of less than 0.1 torr.Alternatively, creating the A_(x)Mn[Fe(CN)₆]_(y).zH₂O paste in Step 902includes creating the A_(x)Mn[Fe(CN)₆]_(y).zH₂O paste in a cubicstructure, and Step 906 dries the electrode in a vacuum oven at apressure of less than 0.1 torr.

In one aspect, forming the A_(x)Mn[Fe(CN)₆]_(y).zH₂O material on thecurrent collector in Step 904 includes substeps. Step 904 a coats thecurrent collector with the A_(x)Mn[Fe(CN)₆]_(y).zH₂O paste. Step 904 bdries the A_(x)Mn[Fe(CN)₆]_(y).zH₂O paste, and Step 904 c presses thecoated current collector. After printing and drying (Steps 904 a and 904b), the electrode is very porous due to solvent evaporation. Theparticles (TMH and conducting carbon) are bonded by organic binder.However, for a battery, the ideal porosity for electrode is smaller, butstill large enough to permit the entry of the electrolyte into theelectrode. The porosity is reduced by pressing is to compact theparticles assembly.

In another aspect, forming the A_(x)Mn[Fe(CN)₆]_(y).zH₂O material on thecurrent collector in Step 904 includes alternative substeps. Step 904 dforms a self-standing film from the A_(x)Mn[Fe(CN)₆]_(y).zH₂O paste.Step 904 e presses the self-standing film on the current collector.

FIG. 10 is a flowchart illustrating a method for using a TMH battery.The method begins at Step 1000. Step 1002 provides a battery with ananode, an electrolyte, and a A_(x)Mn[Fe(CN)₆]_(y).zH₂O cathode. The Acations are alkali or alkaline-earth cations, such as Na or K. Thevariable x is in the range of 1 to 2, y is in the range of 0.5 to 1.0,and z is in the range of 0 to 3.5. The electrolyte may be an organicsolvent containing an A-atom salt. Step 1004 connects a load between thecathode and the anode. Step 1006 discharges the battery in a singleplateau discharge curve, where a single plateau discharge curve isdefined as a constant discharge voltage slope between 85% and 15%battery charge capacity. Alternatively stated, in Step 1006 the singleplateau discharging curve is defined by the derivative of chargecapacity to voltage (dQ/dV) of a completed discharging curve having onlyone peak. In one aspect, Step 1006 discharges with a capacity of greaterthan 90 milliamp-hours per gram (mAh/g).

Subsequent to disconnecting the load in Step 1008, Step 1010 connects abattery charging device between the cathode and the anode. Step 1012charges the battery in a single plateau charge curve, where a singleplateau charge curve is defined as a constant charge voltage slopebetween 85% and 15% battery charge capacity. Alternatively stated, thesingle plateau charging curve is defined by the dQ/dV of a completedcharging curve having only one peak. The battery can be iterativelycharged and discharged. In one aspect not shown, the battery can becharged while still connected to a load.

In one aspect, Step 1002 provides the battery cathodeA_(x)MnFe(CN)₆.zH₂O having a rhombohedral crystal structure withMn^(2+/3+) and Fe^(2+/3+) having the same reduction/oxidation potential.

A transition metal hexacyanoferrate (TMH) battery, TMH batteryfabrication, and TMH battery usage has been provided. Examples ofparticular materials and process steps have been presented to illustratethe invention. However, the invention is not limited to merely theseexamples. Other variations and embodiments of the invention will occurto those skilled in the art.

1-10. (canceled)
 11. A transition metal hexacyanoferrate (TMH) cathodebattery, the battery comprising: a A_(x)Mn[Fe(CN)₆]_(y).zH₂O cathode;where A cations are selected from the group consisting of sodium andpotassium cations; where x is in a range of 1 to 2; where y is in arange of 0.5 to 1; where z is in a range of 0 to 3.5; an anodecomprising hard carbon; and, a non-aqueous electrolyte.
 12. (canceled)13. A transition metal hexacyanoferrate (TMH) cathode battery, thebattery comprising: a A_(x)Mn[Fe(CN)₆]_(y).zH₂O cathode; where A cationsare selected from the group consisting of sodium and potassium cations;where x is in a range of 1 to 2; where y is in a range of 0.5 to 1;where z is in a range of 0 to 3.5; an anode comprising graphite; and, anelectrolyte comprising an organic solvent and A-atom salts. 14.(canceled)
 15. The battery of claim 11 wherein the battery electrodescomprise current collectors made of a material selected from the groupconsisting of aluminum (Al), titanium (Ti), and stainless steel.
 16. Thebattery of claim 11 where the A_(x)MnFe[(CN)₆]_(y).zH₂O has arhombohedral crystal structure with Mn^(2+/3+) and Fe^(2+/3+) having thesame reduction/oxidation potential.
 17. The battery of claim 11 whereinthe TMH cathode has a single plateau charging curve, where a singleplateau charging curve is defined as a constant charging voltage slopebetween 15% and 85% battery charge capacity.
 18. The battery of claim 11wherein the TMH cathode has a single plateau discharge curve, where asingle plateau discharge curve is defined as a constant dischargevoltage slope between 85% and 15% battery charge capacity.
 19. Thebattery of claim 13 wherein the battery electrodes further comprisecurrent collectors made of a material selected from the group consistingof aluminum (Al), titanium (Ti), and stainless steel.
 20. The battery ofclaim 13 where the A_(x)MnFe[(CN)₆]_(y).zH₂O has a rhombohedral crystalstructure with Mn^(2+/3+) and Fe^(2+/3+) having the samereduction/oxidation potential.
 21. The battery of claim 13 wherein theTMH cathode has a single plateau charging curve, where a single plateaucharging curve is defined as a constant charging voltage slope between15% and 85% battery charge capacity.
 29. The battery of claim 13 whereinthe TMH cathode has a single plateau discharge curve, where a singleplateau discharge curve is defined as a constant discharge voltage slopebetween 85% and 15% battery charge capacity.